In recent years, the recording density of hard disk devices (magnetic recording and reproducing devices or HDDs), which are a type of magnetic recording and reproducing device, has been significantly improved and the current recording density of mass-produced hard disk devices reaches 80 Gbpsi (gigabits per square inch). A magnetic head in such a magnetic recording and reproducing device is designed to fly at a constant flying height while the magnetic recording medium is being rotated. That is, during a read/write operation, the magnetic head flies above the magnetic recording medium at a constant flying height to prevent direct contact between the magnetic head and the magnetic recording medium, so that the reliability of the magnetic recording and reproducing apparatus is enhanced.
Recently, the flying height has lowered in accordance with a rise in recording density and fallen to the point of 10 nm at present.
In order to decrease the head flying height, the smoothness (lack of projections) of the surface of the magnetic recording medium is important. Especially, there is known a thermal asperity phenomenon in which a MR head or a GMR head is unable to read a signal due to an instantaneous temperature rise caused by contact with a projection of the surface of the magnetic recording medium present due to the characteristics thereof, and smoothness of the surface of the magnetic recording medium is especially valued as a measure against thermal asperity.
In order to realize such a smooth surface of a magnetic recording medium, a magnetic recording medium substrate for a magnetic recording medium of recent years is made of a glass substrate using chemically tempered glass, crystallized glass or the like instead of aluminum alloy. Since aluminum alloy undergoes plastic deformation due to the material characteristics thereof in the course of mechanical treatment such as a polishing process, it is difficult to obtain the smoothness mentioned above when using aluminum alloy.
In contrast, a glass substrate, which has high hardness of surface and does not undergo plastic deformation even when treatment such as a polishing process is performed, has the advantage that it makes it easy to obtain a smooth surface. For these reasons, hard disks having magnetic recording medium substrates made of glass are being increasingly used as hard disks having high recording density.
A magnetic recording medium substrate made of glass, however, has the following problems.
Regarding glass to be used for the magnetic recording medium substrate, tempered glass includes sodium and crystallized glass includes lithium as a main alkali metal. Such a metal ion, which has a small ionic radius, is easy to move, and is especially easy to move at high temperature and high humidity. Although the mechanism thereof has not been elucidated, sodium ions or lithium ions which move to the surface of a magnetic recording medium react with surrounding substances to form a variety of compounds such as hydroxides and carbonates and form projections on the surface of the magnetic recording medium, bringing the possibility of head crash due to a collision with a head which is flying. Moreover, the possibility of error occurrence during a read/write operation or of head crash as a consequence of deterioration of the characteristics of a magnetic film caused by corrosion of the magnetic film has been pointed out. Furthermore, the possibility of corrosion of a magnetic head element itself or of head crash due to adhesion of these alkali metal ions to the head side has also been pointed out (see Japanese Unexamined Patent Application First Publication No. 2001-23155, for example).